The present invention provides for a novel method for affecting the optical clarity of polycarbonate resin which has been prepared via the transesterification of a diaryl carbonate and a dihydric phenol, which involves adding to the polycarbonate resin a minor but effective amount of a lithium compound nucleating agent.
It is known that various aromatic polyaryl esters or polyarylates may be prepared by transesterification and polycondensation of mixtures of diaryl esters of aromatic carboxylic acids, together with diaryl carbonate, if desired, and dihydric phenols, and that polycarbonates may be prepared by transesterification of a diaryl carbonate with a dihydric phenol. In this application, polyarylates are considered to be polycarbonates in which all or part of the carbonic acid residues are replaced by aryl dicarboxylic acid residues, preferably isophthalic and/or terephthalic acid residues.
A transesterified polycarbonate resin, employable for purposes of this invention, is suitably prepared by blending, e.g., diphenyl carbonate and bisphenol-A as a melt in the presence of an appropriate catalyst. The molecular weight of the resulting resin is preferably in the range between 8,000 to 100,000, preferably 10,000 to 80,000, and exhibits an intrinsic viscosity between 0.4 to 0.7 dl/g.
Conventionally, in carrying out the transesterification process, the reaction components are passed through a number of stages involving melting the reaction components, reacting the components, pre-polymerizing the reaction product and, finally, polymerizing the effluent from the pre-polymerization stage in a final polymerization stage.
Heretofore, in processes employing the aforementioned series of steps, thin film evaporators have been employed only in the final polymerization stage.
One disadvantage with the various processes conventionally employed in the art lies in the fact that the speed of the pre-polymerization step is limited by mass transfer constraints brought about by the formation of byproducts of the reaction, which ultimately necessitates a minimum residence time under the specific reaction conditions in order to effect the degree of product formation desired.
A new method for the transesterification of a diaryl carbonate and dihydric phenol which allows for shorter residence time in the pre-polymerization stage of the reaction process, and thus produces an enhanced yield of reaction product in less time, comprises using a series of equipment including a melter, a reactor, a pre-polymerization reactor and then more polymerization reaction vessels.
The utilization of non-ferrous reaction vessel components, such as, nickel, titanium, chromium metals or the like for all contact surfaces, in such a system, such as for pipings, reactor, and/or recovering vessels, etc., clad or lined with such materials or with glass, provides a superior system for producing polycarbonate resin via the transesterification reaction, since the resultant product will be free from undesirable color formation.
U.S. patent application Ser. No. 176,865, filed Aug. 10, 1980, which is also assigned to the assignee of the present application, discloses such a reaction system and the teachings of that disclosure are also incorporated herein by reference and may be utilized to advantage in preparing the resin to be utilized in the crystallization process of the present invention.
Said application also discloses that carrying out the reaction to produce a polycarbonate resin via the transesterification of a diaryl carbonate and a dihydric phenol, allowing the components to reach equilibrium in an inert atmosphere in the melt-reaction step prior to introducing such partially reacted components to a series of ester-interchange reaction vessels, which are operated under vacuum, wherein the excess phenol produced is removed via distillation and the entrained diphenyl carbonate is refluxed to the ester-interchange reaction vessel, and carrying out the reaction in a totally non-ferrous material environment allows for the production of a product in higher yield and shorter residence time than has heretofore been possible.
In carrying out the transesterification of diphenyl carbonate and bisphenol-A to produce a bisphenol-A polycarbonate, with the removal of phenol, various lithium salts are known to have utility as catalysts for the reaction. Typically, lithium salts such as the various lithium halides and lithium hydroxides have been employed for this purpose. It has been found, however, that the effectiveness of the lithium salts which are conventionally employed for carrying out the transesterification of bisphenol-A and diphenyl carbonate is limited by the fact that such salts are only partially soluble in the reaction medium.
The polycarbonate resins produced via the transesterification of a diaryl carbonate and a dihydric phenol find utility in the production of various films and similar materials, wherein controlled crystallization of the resinous materials is effected via known techniques. Control by crystallinity is important in achieving optical clarity and the best impact properties in films and thicker molded articles.
The present invention provides for a beneficial method for promoting and enhancing the controlled crystallization of polycarbonate resins produced via the transesterification of a diaryl carbonate and dihydric phenol which allows for the production of sheet and film material having improved characteristics over those produced using prior art methods. It is based on the addition of lithium compounds in very small amounts to transesterified polycarbonates.